We have proposed a hypothesis that holds that steroidogenesis occurs in specific biosynthetic units (dubbed hormonosomes), one kind for each of the different steroid hormones. In an effort to obtain further proof for this thesis, we are examining various enzymatic processes which are involved in hormone biosynthesis. The objective is to obtain evidence for the existence of isoenzymes that are predicted by this thesis. We have already examined the cholesterol side-chain cleavage enzymes that occur in homogenates of adrenals and corpus lutea. The evidence is clear that side-chain cleavage is catalyzed by different enzyme systems depending upon the tissue source and the available substrates. Our intention is to examine other enzyme systems, especially aromatase and the 17-hydroxylase-lyase complex in ways, usually kinetic, that may reveal the existence of isoenzymes, i.e., more than one enzyme system for the catalysis of what superficially appears to be the same reaction.